There is a growing need for a small, accurate, sensitive, low cost device for monitoring the amount of radioactive radon daughter products (radon) in air.
Such a monitoring device is needed for uranium miners in order to assure that the concentration of radon and its daughter products in the mine air they breathe is in compliance with Federal exposure limit regulations. There is also a growing need for a similar but more sensitive device which can monitor these radioisotopes in poorly ventilated buildings to protect the health of the inhabitants. Such devices are also needed to measure the radon in soil since higher concentrations indicate the presence of uranium ore buried deeply beneath the surface.
Radon is a gaseous decay product in the uranium decay series which decays by alpha emission with a half-life of 3.5 days. When it is formed near the surface of uranium containing materials such as soil or rocks, it diffuses out into the surrounding air where it can pose a radiological hazard under certain circumstances. Some of the daughter products of radon, which form and decay in sequence soon after the radon decays, are potentially more hazardous to man than their radon parent because of their more energetic emissions. By measuring these radon daughter products with the present invention over a known time period one can readily calculate their concentration in air and that of their radon parent as well.
Several instruments are commercially available for measuring radon and/or its daughter isotopes but they are large and costly and generally unsuited for use in the applications cited above.
A radon measuring device was described by Cost-Ribeiro, et al., in 1969 ("A Radon Detector Suitable for Personnel or Area Monitoring", Health Physics, Vol. 17, pp 193-198) which utilizes the well-known passive diffusion (PD) chamber principle for sample collection and an electrostatic field to attract the charged radon daughters to a detector. However, that earlier device, which is now commercially available, utilizes a large high voltage battery pack to supply the high voltage needed to electrostatically transport and collect the charged atoms toward and on the detector surface. As a result, this device is inherently bulky, heavy and expensive and, accordingly, is not at all practical for use in applications to which the present invention is directed. Moreover, the earlier instrument utilizes an electronic detector to measure the radon daughter radiation in real-time. An improved version of this device is now available which also uses an integrating detector. However, it still uses the bulky battery pack. Still another version now available employs the PD chamber principle and the small alpha track detector (not a TLD detector).